1. Field of the Invention
The present invention relates to an image display device having a memory property, a driving control device and driving method to be used for the same; and more particularly to the image display device having a memory property being suitably used in an electronic paper display device such as an electronic book and electronic newspaper and to the driving control device and driving method to be used for the image display device.
2. Description of the Related Art
As a display device enabling an act of “reading” without the reader feeling stress, an electronic paper display device called an electronic book, electronic newspaper, and the like is under development. It is required that the electronic paper display device of this kind is thin, light-weight, hard to crack and consumes less power and, therefore, it is preferable that the electronic paper display device is made up of a display device having a memory property. Conventionally, as a display element to be used for the display device having a memory property, an electrophoretic element, electronic liquid powder display, cholesteric liquid crystal display device and a like are known. Among them, an electrophoretic display device using a microcapsule-type electrophoretic element is receiving attention.
FIG. 21 is a partial cross-sectional view schematically showing a diagrammatic configuration of an electrophoretic display device of an active matrix driving type. The electrophoretic display device, as shown in FIG. 21, is made up of a TFT (Thin Film Transistor) glass substrate 1, an electrophoretic element film 2, and a facing substrate 3, all of which is stacked in layers in this order. On the TFT glass substrate 1 are mounted many thin film transistors (hereafter “TFTs”) 4 serving as switching elements arranged in a matrix form, pixel electrodes 5 each being connected to each of the TFTs 4, gate lines 6, data lines (not shown), and light shielding films 7 each covering the TFTs 4. The above electrophoretic element film 2 is made up of microcapsules 9, 9, . . . being about 40 μm in size which are spread over a polymer binder 8. Each of the microcapsules 9, 9, . . . is filled with a solvent 10. In the solvent 10 are trapped, in a manner to be spread and to allowed to float, an infinite number of positively or negatively charged nano-sized particles, that is, white pigment particles 11, 11, . . . such as negatively charged titanium oxide particles and black pigment particles 12, 12, . . . such as positively charged carbon particles. Moreover, on the above facing substrate 3 is mounted a facing electrode 13 to supply a reference potential.
The electrophoretic display device performs its operations by applying a voltage corresponding to image data between pixel electrodes 5 and facing electrodes 13 and moving white pigment particles 11, 11, . . . and black pigment particles 12, 12, . . . up and down. That is, when a positive voltage is applied to the pixel electrode 5, the negatively charged white pigment particles 11, 11, . . . are attracted toward the pixel electrodes 5, whereas and a positively charged black pigment particles 12, 12, . . . are attracted toward the facing electrode 13 and, therefore, if the facing electrode 13 side is used as a display face, black is displayed on the screen. On the other hand, when a negative voltage is applied to the pixel electrode 5, the positively charged black pigment particles 12, 12, . . . are attracted toward the pixel electrodes 5 and, whereas negatively charged white pigment particles 11, 11, . . . are attracted toward the facing electrode 13 and, therefore, white is displayed on the screen. When an image is to be switched from white display to black display, a positive signal voltage is applied to the pixel electrode 5. When the image is to be switched from black display to black display, a negative signal voltage is applied to the pixel electrode 5. When a present image is maintained, that is, when the image is switched from white display to white display and from black display to white display, a 0V voltage is applied to the pixel electrode 5. Thus, since the electrophoretic display element has a memory property, by comparing a previous screen with a subsequent screen (renewed screen), a signal voltage to be applied is determined.
Next, a TFT driving method for active-matrix type electrophoretic display device is described. In the TFT driving method of the electrophoretic display element, as in the case of a liquid crystal display device, a gate signal is applied to the gate lines 6 to perform a shift operation for every frame and a data signal is written through the TFTs 4 of the switching element to the pixel electrodes 5. Time required for completion of writing of all lines is defined as “one frame” and one frame scanning is performed for, for example, at 60 Hz (=16.6 ms). In general, in a liquid crystal display device, an entire image is switched within 1 frame. On the other hand, the response speed of the electrophoretic element is slower than that of the liquid crystal display device and image switching cannot be made unless a voltage continues to be applied for a plurality of frame periods and, therefore, in the electrophoretic display device, a PWM (Pulse Width Modulation) driving method is used in which a constant voltage continues to be applied for a plurality of frame periods.
In the electrophoretic display device providing a slow response speed, when an image is to be renewed, it is necessary that a history of a previous screen is deleted. In the non-patent reference document 1 (SID Technical Digest [2006, P1406, Improved Electronic Controller for Image Stable Display]), a reset driving method is disclosed in which, to delete a history of a previous screen, after a screen is first reset by displaying black and then white on an entire screen, a renewed screen is displayed.
Next, the reset driving method disclosed in the non-patent reference document 1 is described by referring to FIG. 22. For the convenience of descriptions, it is assumed that the response speed of the electrophoretic display element is, for example, 0.5 sec and a frame frequency is 60 Hz.
In the reset driving method, when image display is to be switched, a voltage (pixel voltage) of +15V is first applied continuously for a period of time corresponding to a response speed of the electrophoretic display element (time corresponding to the response speed), for example, about 0.5 sec to display black. As shown in FIG. 22, a pixel voltage of +15V is continuously applied to the electrophoretic display element for frame period of N1 (hereinafter, N1 frame time). Here, the N1 frame corresponds to 30 frames (500 ms/16.6 ms). After N1 frame time has elapsed, a pixel voltage of −15V is continuously applied to the electrophoretic display element for a period of time corresponding to N2 frame (30 frames) to display white on a screen. Thus, after resetting the entire screen by black and white, an image on a subsequent screen (renewed screen) is displayed with a specified gray level.
The gray level display is performed by applying a voltage of +15V for a period of time defined according to a gray level of a subsequent screen (renewed screen) within a period of time corresponding to N3 frames (30 frames). That is, when white is to be displayed (with 15th gray level) on a subsequent screen, white has already been displayed on the previous screen and, therefore, no voltage is applied on the subsequent screen. When black is to be displayed (with 0th gray level) on a subsequent screen, a voltage of +15V is continuously applied for periods (30 frames) corresponding to the response speed of the electrophoretic display element. Moreover, the display of an image with an intermediate gray level can be realized by decreasing count of frames for which a voltage of +15V is continuously applied according to gray level (luminance). That is, when an image is to be displayed with 14th gray level on a subsequent screen, a voltage of +15V is applied for a period of time corresponding to 2 frames and, when an image is to be displayed with 13h gray level on the subsequent screen, a voltage of +15V is applied for a period of time corresponding to 4 frames, and when an image is to be displayed with (15−n)th gray level on the subsequent screen, a voltage of +15V is applied for a period of time corresponding to 2n frames, and further when an image is to be displayed with 1st gray level on the subsequent screen, a voltage of +15V is applied for a period of time corresponding to 28 frames.
In the reset driving method, due to necessity of display of a redundant reset screen, there is a fear of degrading the display performance. To solve this problem, a previous screen reference driving method is disclosed in which a voltage to be applied is determined by using a look up table (Look Up Table, hereinafter simply an LUT) being a table showing a specified conversion coefficient group used to calculate a data signal from gray level data of a previous screen and gray level data of a renewed screen.
However, the previous screen reference driving method has a shortcoming in that, the reset screen display can be omitted at time of screen renewal and, as a result, the method is excellent in display performance, however, unless the LUT is properly set, a slight previous screen is left, that is, an afterimage phenomenon occurs.
There is, however, another problem in that, as the gray level becomes multiple from 16→32→64 gray levels, the configuration of the LUT becomes the more complicated, which causes a difficulty in adjustment for obtaining an excellent image.
For example, in the previous screen reference method, a voltage has to be determined according to the LUT set for every frame from gray level data of a previous screen and gray level of a subsequent screen. Therefore, it is necessary that the LUT having a group of conversion coefficients (16×16, 32×32, and 64×64) of a previous image (4 bits=16 gray levels, 5 bits=32 gray levels, 6 bits=64 gray levels) and a renewed image (4 bits=16 gray levels, 5 bits=32 gray levels, 6 bits=64 gray levels) corresponding to frames required for renewing driving operations. To satisfy this, a process of determining huge pieces of matrix data is required, thus causing the LUT adjustment required for obtaining an appropriate image to be complicated.
Moreover, there is a contradiction that the improvement of a response speed of the electrophoretic element causes the difficulty in multiple gray level display. For example, the response speed of the electrophoretic element in driving at 15V is improved from 500 ms to 125 ms. In the case of the number of frame frequencies being 60 Hz, for the electrophoretic element having a response speed of 125 ms to achieve screen renewal from white to black, a voltage of +15V has to be continuously applied for a period of time corresponding to 30 frames. However, if the electrophoretic element having the response speed of 125 ms is used, a voltage of +15V is simply applied, under a condition of 125 ms/16.6 ms=7, for a period of time corresponding to 5 frames, which can improve the response property.
However, in the latter case, a display shift from white to black occurs for a period of time corresponding to 7.5 frames. For this reason, there remains an inconvenience problem in that multiple gray level display with 8 gray levels at most can be realized according to the above-mentioned driving method. Thus, another technological problem arises that, in order to achieve display with 16 gray levels, a frame frequency has to be raised from 60 Hz to 300 Hz, which causes a rise in power consumption and insufficient writing of signals to a data driver or TFT, as a result, making it impossible to be used in high-definition panel. On the other hand, it can be envisioned that a response speed is made slow by lowering the driving voltage from 15V to 8V, however, the effort of having improved the response speed of the electrophoretic element proves fruitless.